If you want to see what's going on when Drupal's cron.php script runs, one thing you can do is to set a breakpoint in a module's implementation of the hook_cron() function, and call cron.php directly from your debugger. There you can step through your breakpoints, examine variables, and hopefully zero in on your problem.

It’s easy to take Drupal’s built-in user management features for granted… out of the box, you can configure your site to allow user registrations, reset forgotten passwords, and handle user logins. We had a use case where we wanted to combine the login and registration forms into a single landing page for anonymous users, to encourage users without an account to sign up with a single click rather than hiding the registration form behind another link.